Fast recovery high-mean-power pre-tr switch



RECEIVER TR SWITCH FIG. I.

swlTcH FIG. 2.

PRE-TR Feb. 24, 1970 H, GOLDIE ETAL FAST RECOVERY HIGH-:.iEAN-POWER PRE-TR SWITCH Filed Nov. 23. 1966 CIRCULATOR TRANSMITTER I AToRNri United States Patent O U.S. Cl. S33-13 2 Claims ABSTRACT F THE DISCLOSURE A TR type switch is described which is intended to be inserted in a waveguide coupling a radar receiver to a circulator which is also coupled to a transmitter and a common antenna. The device is intended to be inserted in the waveguide between the transmitter and the conventional TR switch to supplement the protection that the latter provides for the receiver. The switch described herein comprises a tuned iris aperture in an iris plate which is in intimate heat transfer relation with a block of good heat conducting material which has an opening therein to match the opening in the waveguide in which forms a part thereof. In conventional manner, the waveguide is pressurized with an insulating gaseous medium to prevent high voltage arcing. The lips of the iris aperture are therefore immersed in this gaseous insulating medium. The iris plate has one plane side and the other side is provided with a taper in the vicinity of the edges of the aperture to produce thin edges and create a high voltage gradient at the lips of the iris aperture. In proximate and good heat transfer relation to the plane side of the iris plate, a quartz capsule, transparent to microwave energy, encapsulates a fast recovery -gaseous medium, such as a halogen gas which when ionized constitutes a low resistance, high capacitance path across the iris aperture. The design is such that any arcing which might take place will take place in the fast recovery gaseous medium, which although very corrosive is not in contact with any metal parts.

This invention relates, in general, to discharge devices, and more particularly, to gaseous discharge devices for switching ultra high frequency microwave energy. Such devices are commonly known as transmit-receive or TR switches.

Microwave switches of the TR type are well-known and are commonly used in radar systems to permit the time sharing of a single antenna which is used alternately to transmit and receive microwave energy. These switches short circuit the receiver during the pulses from the transmitter and thus prevent damage to the sensitive receiver.

Such a TR switch must change its characteristics very rapidly from a nearly non-conductive state to a conductive state when the transmitter pulse wave front appears, and must change back into the non-conductive condition with great rapidity when the transmitted pulse has terminated. In order to accomplish this, it is necessary that the gaseous medium rapidly ionize so that a space charge may be initiated by a relatively small voltage rise, that is, it should have a comparatively low breakdown voltage. It should also deionize rapidly when the voltage is removed so that the echo return signal may be admitted to the receiver.

The TR switch (sometimes referred to as a pre-TR switch) of the general type to which the present invention relates is described and claimed in U.S. Patent 3,208,012, issued to Gerald I. Klein on Sept. 25, 1965, which patent is assigned to the assignee of this invention. The TR 3,497,833 Patented Feb. 24, 1970 switch of that patent has circular hollow electrodes with a capillary tube containing an ionizable gas extending through the hollow electrodes so that the zone of ionization is in proximate relation to the gap between the electrodes.

The requirement for fast recovery TR devices which operate at relatively high average microwave power is mainly due to the requirements of pulse Doppler airborne radar systems which must operate at a very high pulse repetition rate. The high pulse repetition rate used in these systems allow only a few microseconds for the receive listening period. Consequently, the recovery time of the TR device should not exceed a few tenths of a microsecond.

The combination of high mean power and short recovery period cannot be easily compromised in radar systems. With a gas other than a halogen, the recovery time increases rapidly with mean power. However, halogen gases have high arc losses but this is not particularly significant as regards receiver protection although it does have a bearing on the manner in which the gas is kept in a condition to respond to the rise in voltage. The capture coefficient for free electrons in halogens is large and results in extremely short recovery time. However, halogens are very active and corrosive to metals and therefore electrodes cannot be immersed directly in the discharge medium. Therefore, the gas is encapsulated in a structure such as quartz which is relatively inert to halogens. This necessitates considerations of the best geometry to eiliciently and effectively couple the radio frequency energy into the gas volume in order to ionize the gas and sustain the discharge. It is also important to dissipate the heat energy which otherwise may cause deleterious operation if the device becomes excessively heated. No metallic electrodes will remain unaltered in the ionized gaseous medium when an attacking gas, such as chlorine, is employed.

Because the halogen molecules have such a high capture coefficient, large amounts of energy are required to maintain enough free ions in the high isolation state. As a result, a high average incident power is required with high PRR to develop the extremely high electron temperatures necessary to sustain the discharge in the encapsulated structure. The power density may be as high as 2.3 kilowatts per square centimeter to achieve a stable discharge resulting in power dissipation leading to excessive temperatures. It is the adequate removal of the excessive heat dissipated which is critical in the operation of this invention.

In accordance with the present invention a geometry and construction is provided which assures development of a high potential gradient to ionize the gas and at the same time provide the necessary thermal paths to transfer the heat out of the critical location. The thermal circuits in this invention do not lead to a bulky geometry or special liquid or air cooling.

In general, the present invention utilizes a tuned iris having tapered iris lips which produce the high voltage gradient and at the same time provide thin metal edges to effectively couple these gradients into the gas volume to be ionized. These same tapered irises provide the thermally conductive paths to the heat sink.

Accordingly, the primary object of the invention is to provide a novel and improved electron discharge device of the TR type.

Another object is to provide a TR switch capable of withstanding and handling very high mean power and being capable of returning to the con-conductive state at a very rapid rate.

Other and further objects will be apparent from the following description when taken in consideration with the accompanying drawings, in which:

FIGURE l is a schematic representation of a partial waveguide circuit of a common radar system illustrating the environment for the present invention;

FIG. 2 is an exploded View of components of the present invention;

FIG. 3 is an enlarged partial sectional profile view; and

FIG. 4 is a partial cross-sectional View on the line IV-IV of FIG. 3 looking in the direction of the arrows.

Briey, the present invention comprises a TR type switch of such geometry and construction associated with a heat exchange means which is integral with the means for obtaining a voltage gradient to be developed in the gaseous medium to cause the formation of a low resistance electrical path in the quartz envelope. The gaseous medium will recover to its non-conducting state Very rapidly and thus make it possible to pass electromagnetic energy with a minimum of power loss.

The fast recovery gaseous media, and particularly the one used with this invention, inherently have high capture coefficients for free electrons. In order to keep such gaseous media in a condition for instantaneous breakdown, it is necessary to have a dopant, such as cobalt 60, in the gaseous encapsulated structure to supply some free electrons, which are then excited by the energy of radio frequency eld coupled across the iris so that the ionization process instantly responds to the high voltage condition. This inherently leads to a high average power loss around the discharge area and also at the lips of the iris which, as seen later, are tapered in order to increase the thermal conductance.

The tapering of the edges of the iris, while increasing the potential gradient, increases the transfer of heat and thus solves the problem of (l) maintaining a high potential gradient, (2) supplying the necessary sustaining iield to maintain a stable discharge and (3) extracting the heat from the area of the gaseous medium and its enclosure.

Referring to the drawing, FIGURE l illustrates an environment for use of the present invention and a radar system in which a part of the waveguide circuit includes a transmitter 1 connected by waveguide to a circulator 2 between the transmitter and receiver. The circulator 2 has a waveguide arm 4 connected to the antenna. A waveguide arm 6 connects the circulator with a receiver 7. A

conventional TR switch 8 is in the arm 6 between thev receiver and a pre-TR device 9 in accordance with the present invention inserted in the arm 6 next to the circulator 2 and the TR switch 8.

The device of the present invention includes what might be referred to as a section of waveguide which is in reality a rectangular block 16 of good heat conducting material, such as brass, having a rectangular passage 17, the walls of which constitute a waveguide section. This block along with the other components constitute the TR switch 9 of the present invention. The left-hand end of the TR switch is connected in its usual manner by a bolted flange to the waveguide arm 6 with the opening 17 in the block 16 aligned with the passage in the waveguide. Similarly, the right-hand end of the TR switch is secured by a bolted ange 19 to a waveguide section 21 which leadS to the receiver. The waveguide is pressurized internally at the iris gap with an insulating gaseous medium, such as nitrogen in order to avoid arcing inside the iris gap when operating at relatively high voltages or, in particular, where the equipment is airbourne and may operate at the low atmosphere pressure at high altitudes. Consequently the waveguide components internal to the TR switch are immersed in this insulating gaseous medium.

The TR switch of the present invention comprises an iris 22 extending across the rectangular opening 17 in the block 16. The iris 22 is formed in an iris plate 23 which is between the block 16 and the section of the waveguide leading to the receiver 7. As clearly indicated in FIG. 3 the lips 24 of the iris are tapered on the side facing the incident radio-frequency power, that is, the side facing the waveguide arm. 6. This side 13 in the pressurized system which will be filled with a gaseous medium, such aS nitrogen, capable of withstanding very high potentials. It is not intended that the gap between the lips 24 arc over at any time during the operation since the gaseous medium in which the lips are immersed does not have a fast recovery and this would defeat the operation of the TR switch. It is also true that the potential gradients developed in the iris gap can lead t0 unwanted breakdown; this is prevented by pressurizing this gap as stated above.

A gas capsule 26 is mounted in good heat conducting contact with the plain side of the iris plate 23. The mating surfaces are specially ground for this purpose. The capsule 26 is preferably made of quartz, which is substantially transparent to microwave energy, has a relatively low dielectric coeicient and is relatively insensitive to thermal shock. Another important purpose of the capsule 26 is to confine an atmosphere of the halogen gas in close proximity to the lips of the iris where the ionized gas forms a low resistance path bridging the gap between the lips while at the same time preventing the gas from reacting with the metal.

Since the high power density at which these devices operate produce excessively high temperatures, failure to transfer the heat from the gas plasma-iris region will result in failure of the TR switch. Also any separation of the capsule 26 from the iris plate due to heating or vibration will cause detuning resulting in random arcing leading to local hot spots in the iris. The local hot spots will cause erosion and over-heating of the quartz capsule, both of which in turn cause Some quartz outgassing thus contaminating the halogen gas in the capsule 26. This will increase the recovery time rendering the device ineffective. Also, such heating of the organic adhesives, shown in FIGS. 2 and 3, would cause them to outgas and the inside of the waveguide to be coated which results in increased insertion loss.

In order to properly dissipate and transfer the heat from the lips 24 of the iris in the immediate vicinity of the gap it is necessary that there be an extremely efficient heat transfer mating of the capsule 26 with the plane surface of the iris plate 23 in order to provide a very excellent heat transfer relation. Any adhesive which tends to separate these mating surfaces will reduce the heat flow. Accordingly, the novel construction of the present invention provides an effective means for holding the capsule 26 in good heat transfer relation to the iris plate without having any adhesive between the mating surfaces.

This is accomplished by providing a thin plate, or shim 27, between the iris plate 23` and the block 16 to provide ledges 28 parallel to the end faces of the capsule 26 to which the adhesive 29 may be adhered. It is another purpose of the shim 27 to allow the iris plate 23 to be surface ground for near perfect mating of surfaces. It is clear from FIGS. 3 and 4 that the capsule is slightly smaller than the rectangular passage in the block 16 forming part of the electrical circuit. The adhesive 29 is placed between the end faces of the capsule 26 and the inner side surfaces or ledges 28 of the rectangular opening in the shim 27. In this novel manner capsule 26 is held firmly against the ground surface of the iris plate 23 without any chance of a tilm of adhesive getting between the mating surfaces and thus reducing the heat transfer.

In constructing the present device the quartz capsule 26 which lcontains the halogen gas has one of its broad faces 26 ground at. The iris plate 23, the body of which is brass is gold plated to prevent oxidation and tarnishing under the high temperature conditions and the ground surface 26a of the capsule 26 is mated against the plane surface side 23a of the iris plate. Next the shim 27, which is also made of gold plated brass is bolted in place against the surface 23a of the iris plate 23 to form the recesses between the ends of the capsule 26 and the rectangular opening in the shim 27 to provide the ledges to which the adhesive may be adhered when it is placed in the recesses. As best seen in FIGS. 3 and 4 the adhesive 29 is between the end faces of the capsule 26 and the end faces of the rectangular opening in the shim 27. The capsule 26 is held tightly against the surface of the iris plate in the properly centered position and controlled quantities of adhesive are placed at either end of the capsule. The adhesive is then directed to a heating cycle which consists of heating for periods of one, two, two, one, and eight hours, respectively, at temperatures of 65 C., 100 C., 150 C., 200 C., and 250 C. Thereafter, the assembly is subjected to dielectric heating for approximately four hours at high radio-frequency power levels. The cement used has good radio-frequency properties, bonds very rmly but is not excessively rigid and brittle and results in long adhesive life. The quartz capsule has been previously heated to white heat for minutes and 900 C. for 24 hours to insure no contamination of the halogen gas by impurities trapped in the walls.

A typical set of data in X-band frequencies at high duty cycle at 200 watts average power is as follows:

Recovery tirne=.025 microsecond Isolatiorr=22 db Firing power= watts peak Arc loss=2 db We claim as our invention:

1. A microwave switching device, comprising a section of hollow waveguide through which microwave energy may be propagated, said section being pressurized by a first ionizable gaseous insulating medium, a tuned narrow transverse iris aperture in the form of a narrow transverse slot disposed within said waveguide section in an iris plate extending transversely across said section and in good heat transfer relation with said waveguide, means for confining a tbody of a second gaseous medium which when in un-ionized condition presents very high impedance and when ionized by radio-frequency energy in said waveguide abruptly changes its impedance to provide high isolation at said iris by reason of the capacitive coupling between said body of gaseous medium and said iris plate, the edges of said slot being tapered to thereby increase the voltage gradient sufficiently to break down said body of said second gaseous medium, said iris plate having one plane face secured in heat conducting relation to said waveguide section and the other face being tapered toward the edges of said slot, said means for confining said body of said second gaseous medium being a capsule transparent to microwave energy in good heat transfer relation with said iris plate, said capsule being in intimate direct contact with the plane face of said iris plate, a shim plate attached to one side of said iris plate and having an opening surrounding said capsule, said capsule being bonded to said shim plate by adhesive between the outer surfaces of said capsule and the inner surfaces of the opening in said shim.

2'. The combination as set forth in claim 1, in which said second gas is a halogen.

References Cited UNITED STATES PATENTS 2,422,190 6/1947 Fiske 333-13 XR 2,773,243 12/1956 Goldstein et al. 315-39 XR 2,776,409 1/1957 Goldstein 333-13 2,791,720 5/1957 Lesch 333-13 XR 3,151,275 9/1964 Maddix 333-13 XR 3,268,757 8/1966 Woerrnbke 315--39 XR 3,311,852 3/1967 Giller S33-13 3,337,819 8/1967 Schreyer 333-13 XR FOREIGN PATENTS 1,229,387 3/1960 France.

OTHER REFERENCES Microwave Transmission Circuits, Ragan, McGraw- Hill, New York, 1948, TK6553 R34; pp. 139-140 relied upon.

HERMAN KARL SAALBACH, Primary Examiner T. VEZEAU, Assistant Examiner U.S. Cl. X.R. 315-39; 333-17 

